wait: fix reparent_leader() vs EXIT_DEAD->EXIT_ZOMBIE race
authorOleg Nesterov <oleg@redhat.com>
Mon, 7 Apr 2014 22:38:41 +0000 (15:38 -0700)
committerBen Hutchings <ben@decadent.org.uk>
Wed, 30 Apr 2014 15:23:26 +0000 (16:23 +0100)
commit dfccbb5e49a621c1b21a62527d61fc4305617aca upstream.

wait_task_zombie() first does EXIT_ZOMBIE->EXIT_DEAD transition and
drops tasklist_lock.  If this task is not the natural child and it is
traced, we change its state back to EXIT_ZOMBIE for ->real_parent.

The last transition is racy, this is even documented in 50b8d257486a
"ptrace: partially fix the do_wait(WEXITED) vs EXIT_DEAD->EXIT_ZOMBIE
race".  wait_consider_task() tries to detect this transition and clear
->notask_error but we can't rely on ptrace_reparented(), debugger can
exit and do ptrace_unlink() before its sub-thread sets EXIT_ZOMBIE.

And there is another problem which were missed before: this transition
can also race with reparent_leader() which doesn't reset >exit_signal if
EXIT_DEAD, assuming that this task must be reaped by someone else.  So
the tracee can be re-parented with ->exit_signal != SIGCHLD, and if
/sbin/init doesn't use __WALL it becomes unreapable.

Change reparent_leader() to update ->exit_signal even if EXIT_DEAD.
Note: this is the simple temporary hack for -stable, it doesn't try to
solve all problems, it will be reverted by the next changes.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Reported-by: Jan Kratochvil <jan.kratochvil@redhat.com>
Reported-by: Michal Schmidt <mschmidt@redhat.com>
Tested-by: Michal Schmidt <mschmidt@redhat.com>
Cc: Al Viro <viro@ZenIV.linux.org.uk>
Cc: Lennart Poettering <lpoetter@redhat.com>
Cc: Roland McGrath <roland@hack.frob.com>
Cc: Tejun Heo <tj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

index 234e152..fde15f9 100644 (file)
@@ -734,9 +734,6 @@ static void reparent_leader(struct task_struct *father, struct task_struct *p,
                                struct list_head *dead)
        list_move_tail(&p->sibling, &p->real_parent->children);
-       if (p->exit_state == EXIT_DEAD)
-               return;
         * If this is a threaded reparent there is no need to
         * notify anyone anything has happened.
@@ -744,9 +741,19 @@ static void reparent_leader(struct task_struct *father, struct task_struct *p,
        if (same_thread_group(p->real_parent, father))
-       /* We don't want people slaying init.  */
+       /*
+        * We don't want people slaying init.
+        *
+        * Note: we do this even if it is EXIT_DEAD, wait_task_zombie()
+        * can change ->exit_state to EXIT_ZOMBIE. If this is the final
+        * state, do_notify_parent() was already called and ->exit_signal
+        * doesn't matter.
+        */
        p->exit_signal = SIGCHLD;
+       if (p->exit_state == EXIT_DEAD)
+               return;
        /* If it has exited notify the new parent about this child's death. */
        if (!p->ptrace &&
            p->exit_state == EXIT_ZOMBIE && thread_group_empty(p)) {